Eliza (Helen Slater) isn’t just visiting Kara (Melissa Benoist) and Alex (Chyler Leigh) to celebrate Thanksgiving Day with them in tonight’s episode of “Supergirl.” She is also going to lend a hand in the investigation of an alien mass murder caused by the mysterious virus, Medusa.

In the “Inside: Supergirl” featurette for Season 1, episode 8, Alex asks J’onn (David Harewood) if she can bring her mom to the DEO to help them solve the case. “I have never heard of a weapon, bio or otherwise that can target only alien physiology. That thing killed everything in that bar except human beings,” Alex says of Medusa. “I would like to bring my mother to help analyze the crime scene. She specializes in Astrobiology. I think she could really help.”

The bar Alex is referring to is the alien dive bar where M’gann (Sharon Leal) works. In a sneak peek for the episode, Cyborg Superman (Harewood) — a.k.a. the real Hank Henshaw — unleashes a gaseous substance in the pub that instantly kills several of its alien customers. The substance is believed to contain the virus Medusa, whose recipe is obtained by Cyborg Superman by gaining access to the Fortress of Solitude last episode.

The featurette also shows the arrival of Barry (Grant Gustin) and Cisco (Carlos Valdes) on Kara’s Earth. As the synopsis for the episode revealed, the episode kicks off the four-night crossover event between “Arrow,” “The Flash,” “Supergirl” and “Legends of Tomorrow.”

“What we know from ‘Flash’ is that they need to fight the Dominators who are an alien group hell-bent on killing metahumans,” “Supergirl” executive producer Ali Adler explains in the featurette. “Flash knows that Supergirl owes him a favor [after helping her take down Silver Banshee and Livewire last season], and [so he] comes to knock on our door, and that will kick off the huge week of crossover action fun.”

“Supergirl” Season 2, episode 8 airs tonight at 8 p.m. EST on The CW.

In the “Inside: Supergirl” featurette for Season 1, episode 8, Alex (Chyler Leigh) asks J’onn (David Harewood) if she can bring her mom Eliza (Helen Slater) to the DEO to help solve the alien mass murder caused by the mysterious virus, Medusa.Photo: The CW